1. Field of the Disclosure
This invention pertains to a photosensitive element and a method for preparing a printing form from the photosensitive element, and in particular, to a photosensitive element that is a printing form precursor useful for forming a printing form suitable for relief printing.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials ranging from corrugated carton boxes to cardboard boxes and to continuous web of plastic films. Flexographic printing plates are used in relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. Photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. Photopolymerizable elements are characterized by their ability to crosslink or cure upon exposure to actinic radiation.
Photopolymerizable elements undergo a multi-step process to be converted to a flexographic relief printing form. The photopolymerizable element is imagewise exposed with actinic radiation through an image-bearing art-work, such as a photographic negative, transparency, or phototool (e.g., silver halide films) for so called analog workflow, or through an in-situ mask having radiation opaque areas that had been previously formed above the photopolymerizable layer for so called digital workflow. The actinic radiation exposure is typically conducted with ultraviolet (UV) radiation. The actinic radiation enters the photosensitive element through the clear areas and is blocked from entering the black or opaque areas of the transparency or in-situ mask. The areas of the photopolymerizable layer that were exposed to the actinic radiation crosslink and harden; and, the areas of the photopolymerizable layer that were unexposed, i.e., areas that were under the opaque regions of the transparency or the in-situ mask during exposure, are not cross-linked or hardened, and are removed by treating with a washout solution or with heat leaving a relief image suitable for printing. After all desired processing steps, the printing form is then mounted on a cylinder and used for printing.
Analog workflows involve making an intermediate, i.e., the photographic negative, transparency, or phototool. Preparation of a phototool, such as from a silver halide film, is a complicated, costly and time-consuming process that can require separate processing equipment and chemical development solutions. Alternatively, a phototool can also be prepared from thermal imaging films, or by inkjet methods. Also, quality issues can arise with the use of phototool since the phototool may change slightly in dimension due to changes in temperature and humidity, and all surfaces of the phototool and the photopolymer plate should be clean and free of dust and dirt. The presence of such foreign matter can cause lack of intimate contact between the phototool and plate as well as image artifacts.
An alternative to analog workflow is termed digital workflow, which does not require the preparation of a separate phototool. Photosensitive elements suitable for use as the precursor and processes capable of forming an in-situ mask in digital workflow are described in U.S. Pat. Nos. 5,262,275; 5,719,009; 5,607,814; 6,238,837; 6,558,876; 6,929,898; 6,673,509; 6,037,102; and 6,284,431. The precursor or an assemblage with the precursor includes a layer sensitive to laser radiation, typically infrared laser radiation, and opaque to actinic radiation. The infrared-sensitive layer is imagewise exposed with laser radiation of a digital imager unit whereby the infrared-sensitive material is removed from, or transferred onto/from a superposed film of the assemblage, to form the in-situ mask having radiation opaque areas and clear areas adjacent the photopolymerizable layer. Conventionally, the precursor is exposed through the in-situ mask to actinic radiation in the presence of atmospheric oxygen (since no vacuum is needed). Due in part to the presence of atmospheric oxygen during imagewise exposure, the flexographic printing form has a relief structure that is different from the relief structure formed in analog workflow (based upon the same size mask openings in both workflows). Digital workflow creates a raised element (i.e., dot or line) in the relief structure having a surface area of its uppermost surface (i.e., printing surface) that is significantly less than the opening in the in-situ mask corresponding to the relief structure, depending on the specific precursor chemistry and actinic radiation irradiance. Digital workflow results in the relief image having a different structure for raised elements that print small dots (i.e., raised surface elements) that is typically smaller, with a rounded top, and a curved sidewall profile, often referred to as dot sharpening effect. Dots produced by analog workflow are typically conical and have a flat-top. The relief structure formed by digital workflow results in positive printing properties such as, finer printed highlight dots fading into white, increased range of printable tones, and sharp linework. As such, the digital workflow because of its ease of use and desirable print performance has gained wide acceptance as a desired method by which to produce the flexographic printing form. But not all end-use applications view this dot-sharpening effect as beneficial.
It is known by those skilled in the art that the presence of oxygen (O2) during exposure in free-radical photopolymerization processes will induce a side reaction in which the free radical molecules react with the oxygen, while the primary reaction between reactive monomer molecules occurs. This side reaction is known as inhibition (i.e., oxygen inhibition) as it slows down the polymerization or formation of crosslinked molecules. Many prior disclosures acknowledge that it is desirable for photopolymerization exposure to actinic radiation to occur in air (as is the case for digital workflow), under vacuum (as is the case for analog workflow), or in an inert environment. As disclosed in U.S. Pat. No. 8,241,835, conventional digital workflow has been modified in which imagewise exposure of a precursor occurs in an environment having an inert gas and a concentration of oxygen less than atmospheric oxygen but greater than a completely inert gas environment, i.e., the concentration of oxygen is between 190,000 parts per million (ppm) and 100 ppm. The modified digital workflow provides ease of use of digital workflow while avoiding the dot-sharpening effect of the relief features associated with conventional digital workflow to create relief features having an analog-like appearance.
Additionally, it is often desirable for the flexographic relief printing form to print images, particularly solid areas, with uniform, dense coverage of ink, so-called solid ink density. Poor transfer or laydown of ink from the printing form to the substrate, especially in large areas, results in print defects, such as mottle and graininess. Unsatisfactory printing results are especially obtained with solvent-based printing inks, and with UV-curable printing inks.
There are a number of ways to try and improve the ink density in solid areas of an image printed by a flexographic relief printing form. One way to improve solid ink density is to increase the physical impression between the printing form and the substrate. While this will increase solid ink density, the increased pressure will tend to deform smaller plate elements resulting in increased dot gain and loss of resolution. Another method of improving solid ink density involves increasing the surface area of the relief printing form, since a relief printing form with a roughened surface may hold and thus transfer to the substrate more ink than a smooth surface, and may result in a more uniform appearance. However, the surface roughness should be sufficient to increase ink transfer but not so much as to cause discreet features to directly print as this would result in undesirable artifacts in the final print. Typically a printing form that includes a matted layer and is prepared by analog workflow successfully retains the roughened surface, but in some instances there can be some loss of the fine structure of the roughened surface when prepared by conventional digital workflow because of the dot sharpening effect.
Solid screening is a well-known process for improving the solid ink density in flexographic printing. Solid screening consists of creating a pattern in the solid printing areas of the relief printing form which is small enough that the pattern is not reproduced in the printing process (i.e., printed image), and large enough that the pattern is substantially different from the normal, i.e., unscreened, printing surface. A pattern of small features that is used for solid screening is often referred to as a plate cell pattern or a microcell pattern.
GB 2 241 352 A discloses a process for preparing photopolymer plates having a plurality of well-like depressions by exposing the photopolymer layer to actinic radiation through a photographic mask containing optically transparent areas and optically opaque image areas, and a screen having a plurality of opaque discrete dots or other geometric shapes onto a photopolymer plate and developing the plate, to form a plurality of depressions in the relief planar surface of the exposed portions of the photopolymer layer.
Samworth in U.S. Pat. No. 6,492,095 discloses a flexographic printing plate having solid image areas which are covered by a plurality of very small and shallow cells. The cells are created via a screened film halftone negative, an intermediate photomask, or via a top layer on the plate that is used as a mask.
Currently, various microcell patterns are widely used to improve the capability of relief printing forms to print solids with uniform, dense coverage of ink, i.e., solid ink density. The microcell patterns may be used in solid areas to improve printed ink density, as well as for text, line work, halftones, that is, any type of image element where an improvement in ink transfer characteristics is realized. In digital workflow, a microcell pattern is made into a digital file which is used by the digital imager unit to incorporate the pattern of microcells with the formation of the in-situ mask using laser radiation, usually infrared laser radiation. That is, the microcell pattern is formed from the infrared-sensitive layer that forms the in-situ mask. The microcell pattern is effectively superimposed in the digital file on image areas (often solids) where improved solid ink density is desired. Examples of patterns are small “negative” (blocking actinic radiation) features, e.g. a 96% halftone dot at 400 lines per inch, representing an array of approximately 14 micron diameter actinic radiation-blocking dots approximately 64 microns apart; and small “positive” (passing actinic radiation) features much closer together, e.g. a 12% halftone dot at 1400 lines per inch, representing an array of approximately 7 micron diameter actinic radiation-passing dots approximately 18 microns apart. In the latter example of small “positive” features, the effect of oxygen (dot sharpening) associated with conventional digital workflow can impact the ability to hold the microcell patterns in solid printing areas of the relief printing form. Typically, the finer the pattern of microcells, i.e. the smaller the size of each cell and closer the spacing of the cells, that is formed, the better the results. One problem with this method is that the additional cells increase the amount of time for laser imaging by the laser imager unit of the photosensitive element. In order to provide finer microcell patterns, companies that manufacture digital imager units have had to improve the optical resolution of their imagers and improve their imaging software as well. Both aspects substantially increase the cost of the imager and the time needed to image the photosensitive element.
Stolt et al. in US Patent Publication 2010/0143841 disclose a method to increase solid ink density printing capability for a relief printing form through digital patterning of image areas of the precursor. Stolt et al. disclose applying a pattern to all image feature areas in halftone data that is used to produce an image mask, which is then used to convert the precursor into a relief printing form. After processing, the printing form carries a relief image that resolves the pattern in the surface of the relief features, and provides solid relief features to maintain or increase printed solid ink densities. A problem with this method is that it is still essentially an analog workflow since a phototool is created that is then contacted by lamination with the photopolymerizable layer of the precursor.
So a need arises for a relief printing form to meet the increasing demands for print quality to improve the transfer of ink to printed substrate and to print, particularly solid areas, with uniform, dense coverage of ink. It is also desirable for the printing form to have a relief structure capable of printing a full tonal range including printing of fine print elements and highlight dots and thereby providing improved print quality. There is a need for a method that is simple and relatively quick in preparing the relief printing form from a photosensitive printing form precursor, and yet can provide the printing form with a relief structure that improves transfer of ink to the substrate, without detrimental impact to dot gain and/or image resolution. It is desirable that the method utilizes a digital-like workflow for its ease and simplicity that results in the printing form having a relief structure with features necessary for high quality printing, without the additional expense to upgrade or purchase new digital imaging equipment and software and without the loss in productivity for high resolution imaging in order to form microcell patterns.